could a different power cord cure sibilance?

No, IMHO. Why not hook up the stock cord or swap between another component to see if there's any difference?

Better power delivery usually reduces hash, but this is what the GNSC mod is for. Sometimes a different power cord can also reduce hash, but not all power cords work the same way in all systems, so Cruz123's suggestion is a good one.

I generally consider sibilance different than hash. Sibilance comes and goes depending on the recording, whereas hash is always present regardless of the recording (although varying dynamics will reveal or hide hash).

IMO, the GNSC mod is exposing a pre-existing front end flaw, and/or it's exposing flaws in the recordings which have sibilance issues.

I'd consider looking at adjusting cartridge loading, or set-up, or the stylus profile. I noticed a reduction in sibilance after I set up my cartridge with a MintLP Best Tractor, and when I changed from a Benz Micro Ace to a Dynavector XX MK II cartridge.

I think a much simpler explanation is that the capacitive and/or resistive loading of the ARC needs to be adjusted for your cart. I suggest posting on the analog board for advice, giving your cart model and the current settings on the ARC.